1. Field of the Invention
The present invention relates to a radar mounted in a vehicle and used to avoid collisions by measuring the distance and relative velocity with respect to other vehicles, and more particularly to an FM-CW radar capable of preventing malfunctioning of radar due to interference by radar signals from FM-CW radars mounted in other vehicles.
In recent years, traffic deaths due to vehicle collisions, etc., have been increasing year by year with increasing numbers of vehicles on the road. To cope with this situation, it has been proposed to equip vehicles with a low-cost safety system, such as a vehicle-to-vehicle distance monitoring system, that gives the driver an advance warning of an impending collision in order to reduce vehicle collision accidents. The present invention is concerned with improvements in a radar device used in such a safety system.
2. Description of the Related Art
FM-CW radar has been known as a type of radar system that can measure the distance and relative velocity of target objects. Since the FM-CW radar can measure distance and relative velocity using simple signal processing circuitry and allows the transmitter and receiver to be made simple in construction, this type of radar system has been used as a vehicular anticollision on-board radar of which compact size and low cost are particularly demanded.
The principle of FM-CW radar is as follows. The output of an oscillator is frequency-modulated by a triangular wave of several hundred hertz, and the frequency-modulated wave is transmitted. An echo signal from a target is received, and the received signal is frequency-demodulated using the frequency-modulated wave as the local frequency. The received echo wave from the target is shifted in frequency from the transmitted signal (producing a beat frequency) according to the distance between the radar and the target and the Doppler shift due to the relative velocity of the target. The frequency shift serves as a measure of the distance and the relative velocity.
When this type of radar is used as a vehicular onboard radar, since the distance is about 100 meters at the longest and the relative velocity about 100 km/h at most, the maximum frequency deviation to ensure sufficient distance-measuring accuracy would be about 100 MHz, and short-wavelength radiowaves, for example, in the millimeter wave region above 30 GHz, would be suitable for the transmitted frequency in order to ensure sufficient accuracy of relative velocity measurement.
If many vehicles on the road come to be equipped with such FM-CW radars, there can arise situations where radar signals from different FM-CW radars interfere with one another. For example, when a radar-mounted car A and a radar-mounted car B are running side by side, radiating radar signals toward another car, neighboring-car-interference may be caused by both echo signals being received by one or other of the radar-mounted cars. Furthermore, if there is an oncoming car C mounted with a radar radiating a radar signal toward the radar-mounted car B, oncoming-car-interference may occur when the car B receives the echo signal of its own transmitted signal simultaneously with the radar signal from the car C. Such interference causes an erroneous measurement of the distance and relative velocity of the target car.
In conventional millimeter wave FM-CW radars, since frequency modulation with a very large frequency deviation is used, as described above, an AM component having substantially the same frequency component as the modulating signal is inevitably superimposed on the frequency-modulated wave because of the slope of the voltage-controlled oscillator's oscillation frequency versus output power characteristic. This AM component is detected by a mixer (frequency converter). Since the frequency of the triangular wave for the frequency modulation is very close to the frequency of the echo signal frequency-demodulated from the reflected signal from a target object, the reception S/N ratio is degraded, and therefore, the output power of the transmitter has to be increased in order to obtain the required radar range. The noise caused by the above phenomenon is known as FM-AM conversion noise.
To overcome the above shortcoming, there is proposed a so-called switching radar system in which the transmitter wave is switched at an appropriate frequency and only an intermediate frequency based on the switching frequency is extracted using a band-pass filter.